This invention relates generally to a method for detection and analysis of an impurity content in silicon material, and more particularly to a method for detection and analysis of an impurity content in refined metallurgical silicon.
As the world's population become more and more aware of the problems of limited energy sources and environment pollution, the utilization of solar energy has changed from the research stage into industrialization. Solar photovoltaic technology has drawn universal attention, and silicon solar cells have begun to be widely used.
Conventional silicon cell wafers are made on P-type semiconductor substrates of silicon. The P-type wafer is made by doping of boron (0.02˜0.25 ppmw) into ultra-pure silicon materials (better than 8N), so after the crystal growth, the resistivity is controlled within the range of 0.5˜5 Ω.cm. A silicon wafer produced this way has a very high purity and the contents of unwanted impurities can be largely neglected. However, the price of silicon cells using ultra-pure silicon wafers is very expensive, which is a major hurdle that limits wider application of these types of solar cells.
Refined metallurgical silicon is a new type of low-cost silicon solar cell material. Its purity content is 2˜3 N lower than conventional solar cell grade silicon material, that is, only about 5˜6 N as compared to 8+N. Impurities of boron in refined metallurgical silicon is as high as 1˜2 ppmw, and the content of phosphorus is also quite high, generally in the range of 3˜12 ppmw. Boron is a P-type impurity, while phosphorus is an N-type impurity, so the refined metallurgical silicon is a kind of impurity compensation material. In the purification process of refined metallurgical silicon material, a commonly used method is directional crystallization. As the segregation coefficient of boron is 0.8, boron as an impurity will be distributed evenly in the material. Conversely, the segregation coefficient of phosphorus in metallurgical silicon is relatively smaller at about 0.33, making phosphorus as an impurity in refined metallurgical silicon unevenly distributed, and the content of phosphorus shows an exponential growth from one end of the crystal to the other end.
Conventional methods for detection and analysis of impurity contents includes glow discharge mass spectrometry (GDMS) and plasma mass spectrometry (ICP-MS). When used in the detection of impurities in refined metallurgical silicon, these two methods both have the following shortcomings: (1) one can only detect samples of about 1 g, and the distribution of impurities vary significantly, and therefore, it is very difficult to determine an accurate average content level of impurities in a large amount of the refined metallurgical silicon (such as a crystal ingot casting of 240 kg in a furnace); and (2) both conventional methods require expensive equipment, and the preparation of the sample is very complicated, so the cost of using a multi-point sampling detection method is too high and a multi-point sampling detection method could not be practically applied to actual production.
For these reasons, there are no known methods to accurately control the doping and the resistivity when using refined metallurgical silicon to cast crystal ingots for solar cells, leading to very low yield.
Thus, if one can provide a detection and analysis method that accurately detects the impurity content in a large quantity of refined metallurgical silicon material, it would be conducive to determining the doping in the course of making solar cells, so as to control the resistivity and improve the yield.